1. Field of the Invention
The present invention relates to a method of evaluating dispersion degrees of a plurality of aromatic epoxy resins in a resin mixture obtained by mixing the aromatic epoxy resins.
2. Description of the Related Art
Lightweight, high-strength, fiber-reinforced plastics have been widely used in various fields. For example, in the case of using the fiber-reinforced plastic in a member of an aircraft, the fiber-reinforced plastic is required to have excellent heat resistance, impact resistance, etc. It is difficult to prepare a matrix resin having excellent various properties from one epoxy resin. Therefore, a resin mixture is prepared as the matrix resin by mixing a plurality of epoxy resins in combination.
For example, as proposed in International Publication No. WO 2009/107697, a plurality of epoxy resins having different properties are mixed to prepare the resin mixture. This resin mixture exhibits notable properties different from those of each epoxy resin. The resin mixture, superior to a single epoxy resin, can be used as the matrix resin of the fiber-reinforced plastic for the aircraft member.
In a case where the resin mixture is not sufficiently mixed, the epoxy resins cannot be uniformly dispersed. When the resin mixture is poor in the dispersion of the mixed epoxy resins, the structural material containing the resin mixture exhibits non-uniform properties. For example, in a local position of the material, the physical and mechanical properties may be deteriorated, lowering the heat resistance, the mechanical strength, etc.
In addition, in a case where the epoxy resins are not sufficiently mixed with a hardener by kneading, the epoxy resins and the hardener cannot be uniformly dispersed. When the resin mixture is poor in the dispersion of the mixed epoxy resins, the structural material containing the hardened resin mixture exhibits non-uniform properties. For example, in a local position of the material, the physical and mechanical properties may be deteriorated, extremely lowering the heat resistance, the mechanical strength, etc.
Thus, in view of improving the properties of the structural material containing the resin mixture over the entire material, it is necessary to sufficiently disperse the epoxy resins (and the hardener) in the resin mixture. For example, the sufficient dispersion can be achieved by mixing the components for a longer time under a greater stirring force. However, when the kneading is continued even after the epoxy resins (and the hardener) are sufficiently dispersed, the production efficiencies of the resin mixture and the fiber-reinforced plastic may be deteriorated, and unexpectedly the resin mixture may be excessively hardened.
For the above reasons, in view of dispersing the epoxy resins (and the hardener) efficiently and sufficiently without excess mixing time, there is a demand for a method of obtaining information of the dispersion degrees of the epoxy resins in the resin mixture, i.e. a method of evaluating the dispersion degrees of the epoxy resins (and the hardener).
In the past, the dispersion degrees of the mixed resins have generally been evaluated by visual observation. However, in the visual observation, the dispersion evaluation depends on the visual sense of the observer. Therefore, the dispersion degrees of the mixed resins are qualitatively evaluated, and cannot be quantitatively evaluated. Furthermore, for example, in a case where all the mixed epoxy resins are transparent and do not change the color of the resin mixture after the dispersion, it is difficult to evaluate the dispersion degrees of the mixed resins by the visual observation.
The dispersion degrees of the mixed resins can be quantitatively evaluated by analyzing a sea-island structure of the resin mixture using a transmission electron microscope (TEM). In fact, the dispersion degrees can be supposedly evaluated by using the TEM for observing the sea-island structure of the resin mixture and by calculating the area ratio between the sea epoxy resin and the island epoxy resin.
A use of a Fourier transform infrared spectroscopy (FT-IR) for an analysis of epoxy resins in a resin composition is proposed in Japanese Laid-Open Patent Publication No. 2010-197099. Specifically, in this method, the hardening degree of the resin composition is measured by detecting an infrared absorption peak of an aromatic epoxy resin contained in the resin composition. Epoxy groups in the aromatic epoxy resin are opened and reduced with progression of a hardening reaction. Meanwhile, the number of benzene rings in the aromatic epoxy resin is not changed in the hardening reaction. Thus, the progression of the hardening reaction of the resin composition can be evaluated by measuring temporal change of the ratio between the infrared absorption peaks of the epoxy groups and the benzene rings and by detecting the reduction of the epoxy groups.